1. Field of the Invention
The invention concerns a variable-speed three-phase drive and, in particular a variable-speed three-phase drive which includes an asynchronous motor which is adapted to be connected to a three-phase network.
2. Description of the Prior Art
A particular type of three-phase drive is known in which the asynchronous motor includes a slip ring rotor which is connected with the three-phase terminals of a first, uncontrolled rectifier which is in the form of a bridge circuit. In this drive, the aforesaid rectifier is connected with two inverters and a second, uncontrolled rectifier which is also in the form of a bridge circuit. In particular, the latter elements are connected in alternating order in a ring circuit such that the DC terminals of succeeding rectifiers and inverters are connected with each other. Also, in this known drive, the three-phase terminals of the aforesaid inverters are adapted to be fed back to the three-phase network connected to the motor and the three-phase terminals of the two rectifiers are adapted to be connected with each other via a circuit breaker.
The above-described known three-phase drive is disclosed in detail in the German Pat. No. 1,638,646 and in the publication issued by Siemens AG with the title "Untersynchrone Stromrichterkaskade mit automatischer Stellbereichanpassung" (Undersynchronous converter cascade with automatic adaptation of the control range). As described above and indicated in the latter references, the circuitry of the drive is in the form of a so-called switchable undersynchronous converter cascade. Such a cascade, as compared to an undersynchronous converter cascade which cannot be switched, is less expensive and requires reduced reactive-power consumption, particularly in the upper speed range.
In operation, in the lower part of the speed control range, the switch or circuit breaker of the drive is open and only one rectifier is connected to the slip ring rotor. In this operating condition, the two inverters are connected in series via the second rectifier, which has no voltage. In the upper part of the speed control range, the switch of the drive is closed, thereby also connecting the second rectifier to the slip ring rotor. Thus, in this case two parallel frequency converters are formed, each including one-half of a bridge of a different one of the two rectifiers and an inverter.
Switchable undersynchronous converter cascades of the above type are employed particularly in the drive circuitry for driving pumps or blowers, where the rotor current increases approximately as the square of the speed. In the latter cases, the frequency converter, which includes a rectifier and an inverter, must draw a low current and a high voltage at low speed and a high current and a low voltage at high speed. The described switchable undersynchronous converter cascade adapts itself to this current-voltage characteristic in two steps.
It is an object of the present invention to modify the above-described three-phase drive so as to include two motors, while still retaining the advantages attributable to the switchable undersynchronous converter cascade.